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Chemistry 140 Fall 2002
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Yrd. Do. Dr. Burak ESAT
Fatih University
General ChemistryPrinciples and Modern Applications
Petrucci Harwood Herring9th Edition
Chapter 15: Principles of Chemical Equilibrium
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Contents
15-1 Dynamic Equilibrium15-2 The Equilibrium Constant Expression15-3 Relationships Involving Equilibrium Constants
15-4 The Significance of the Magnitude of anEquilibrium Constant15-5 The Reaction Quotient, Q: Predicting the
Direction of a Net Change
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Chemistry 140 Fall 2002
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Contents
15-6 Altering Equilibrium Conditions:Le Chtelliers Principle
15-7 Equilibrium Calculations:Some Illustrative ExamplesFocus On The Nitrogen Cycle and theSynthesis of Nitrogen Compounds
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15-1 Dynamic Equilibrium
Equilibrium twoopposing processestaking place at equalrates.
H2O(l) H 2O(g)
NaCl(s) NaCl(aq)H2O
I2(H2O) I 2(CCl 4)
CO(g) + 2 H 2(g) CH 3OH(g)
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Chemistry 140 Fall 2002
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15-2 The Equilibrium Constant Expression
Forward: CO(g) + 2 H 2(g) CH 3OH(g)
Reverse: CH 3OH(g) CO(g) + 2 H 2(g)
At Equilibrium:
Rfwrd = k 1[CO][H 2]2
Rrvrs = k -1[CH 3OH]
Rfwrd = Rrvrs
k 1[CO][H 2]2
= k -1[CH 3OH]
[CH 3OH]
[CO][H 2]2=
k 1k -1
= K c
CO(g) + 2 H 2(g) CH 3OH(g)k 1
k -1
k 1
k -1
You may refer to Chapter 14for discussion of rates of reaction
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Three Approaches to Equilibrium
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Chemistry 140 Fall 2002
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Three Approaches to the Equilibrium
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Three Approaches to Equilibrium
[CH 3OH]
[CO][H 2]2
K c(1) = 14.2 M -2
K c(2) = 14.2 M -2
K c(3) = 14.2 M -2
[CH 3OH]
[CO][H 2]K c =
[CH 3OH]
[CO]( 2[H2])
0.596 M -1
1.09 M -1
1.28 M -1
1.19 M -1
2.17 M -1
2.55 M -1
CO(g) + 2 H 2(g) CH 3OH(g)k 1
k -1
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Chemistry 140 Fall 2002
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General Expressions
a A + b B . g G + h H .
Equilibrium constant = K c= [A] a[B] b .[G] g[H] h .
Thermodynamic
Equilibrium constant = K eq=(aG)g(aH)h .
(aA)a(aB)b .
aB =[B]cB0
cB0 is a standard reference state
= 1 mol L -1 (ideal conditions)
= B[B]
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15-3 Relationships Involving theEquilibrium Constant
Reversing an equation causes inversion of K . Multiplying by coefficients by a common factor
raises the equilibrium constant to thecorresponding power.
Dividing the coefficients by a common factorcauses the equilibrium constant to be taken tothat root.
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Chemistry 140 Fall 2002
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Combining Equilibrium Constant
Expressions
N2O(g) + O 2 2 NO(g) K c= ?
N2(g) + O 2 N2O(g) K c(2)= 2.7x10 +18
N2(g) + O 2 2 NO(g) K c(3)= 4.7x10 -31
K c= [N 2O][O 2][NO] 2
=[N2][O 2]
[N 2O][N2][O 2]
[NO] 2
K c(2)
1K c(3)= = 1.7x10 -13
[N 2][O 2]
[NO] 2=
[N2][O 2][N2O]=
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Gases: The Equilibrium Constant, K P
Mixtures of gases are solutions just as liquids are.
Use K P, based upon partial pressures of gases.
2 SO 2(g) + O 2(g) 2 SO 3(g) K c = [SO 2]2[O 2][SO 3]
2
[SO 3]=V
nSO3 = RT
P SO3 [SO 2]=V
nSO2 = RT
P SO2
[O2] = V
nO2 = RT
P O2
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Chemistry 140 Fall 2002
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The Equilibrium Constant, K P
2 SO 2(g) + O 2(g) 2 SO 3(g)
K c =[SO 2]2[O 2]
[SO 3] RT
P SO3
2
RT
P SO2 RT
P O2
=
2
= RT P SO3
2P SO2 P O2
2
K c = K P( RT ) K P = K c( RT )-1
K P = K c( RT ) n
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Pure Liquids and Solids
Equilibrium constant expressions do not containconcentration terms for solid or liquid phases of a singlecomponent (that is, pure solids or liquids).
C(s) + H 2O(g) CO(g) + H 2(g)
K c =[H2O]
[CO][H 2] =P H2O
P COP H2 ( RT )-1
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Chemistry 140 Fall 2002
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15-5 The Reaction Quotient, Q: Predicting
the Direction of Net Change.
Equilibrium can be approached various ways. Qualitative determination of change of initial
conditions as equilibrium is approached isneeded.
CO(g) + 2 H 2(g) CH 3OH(g)k 1
k -1
At equilibrium Qc = K cQc = [A] tm[B] tn[G] tg[H] th
mA + nB gG + hH
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Reaction Quotient
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Chemistry 140 Fall 2002
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15-6 Altering Equilibrium Conditions:Le Chtelliers Principle
When an equilibrium system is subjected to achange in temperature, pressure, or concentrationof a reacting species, the system responds byattaining a new equilibrium that partially offsetsthe impact of the change.
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Le Chtelliers Principle
Q = = K c[SO 2]2[O 2]
[SO 3]2 Q > K c
2 SO 2(g) + O 2(g) 2 SO 3(g)k 1
k -1
K c = 2.8x10 2 at 1000K
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Chemistry 140 Fall 2002
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Effect of Temperature on Equilibrium
Raising the temperature of an equilibriummixture shifts the equilibrium condition in thedirection of the endothermic reaction .
Lowering the temperature causes a shift in thedirection of the exothermic reaction.
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Effect of a Catalyst on Equilibrium
A catalyist changes the mechanism of areaction to one with a lower activation energy.
A catalyst has no effect on the condition ofequilibrium. But does affect the rate at which equilibrium is
attained.
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Chemistry 140 Fall 2002
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15-7 Equilibrium Calculations:Some Illustrative Examples.
Five numerical examples are given in the textthat illustrate ideas that have been presented inthis chapter.
Refer to the comments which describe themethodology. These will help in subsequentchapters.
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Predicting the Effect of a Change in Concentrationon the Position of the Equilibrium
Problem: Carbon will react with water to yield carbon monoxide andand hydrogen, in a reaction called the water gas reaction that was usedto convert coal into a fuel that can be used by industry.
C (s) + H 2O (g) CO (g) + H 2 (g)What happens to:(a) [CO] if C is added? (c) [H
2O] if H
2is added?
(b) [CO] if H 2O is added? (d) [H 2O] if CO is removed?Plan: We either write the reaction quotient to see how equilibrium willbe effected, or look at the equation, and predict the change in directionof the reaction, and the effect of the material desired.Solution: (a) No change, as carbon is a solid, and not involved in the
equilibrium, as long as some carbon is present to allow the reaction.(b) The reaction moves to the product side, and [CO] increases.(c) The reaction moves to the reactant side, and [H 2O] increases.(d) The reaction moves to the product side, and [H 2O] decreases.
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Chemistry 140 Fall 2002
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Synthesis of Ammonia
The optimum conditions areonly for the equilibriumposition and do not take intoaccount the rate at whichequilibrium is attained.
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Chapter 15 Questions
Develop problem solving skills and base your strategy noton solutions to specific problems but on understanding .
Choose a variety of problems from the text as examples.
Practice good techniques and get coaching from people whohave been here before.